Use a Digital Multimeter

A multimeter, also called a volt-ohm meter or VOM, is a device that measures resistance, voltage and current in electronic circuits. Some also test diodes and continuity. Multimeters are small, lightweight and battery operated, and they can test a wide variety of electronic components in many situations, making them indispensable tools for anyone who needs to test and repair electronic circuits.

Steps

Measuring Resistance

  1. Connect the multimeter to the circuit. Insert the black probe into the common terminal and the red probe into the terminal marked for measuring volts and ohms. The terminal may also be marked for testing diodes.
  2. Twist the selector knob to set the multimeter to measure resistance. This may be represented by the Greek letter Omega, which stands for ohms, the unit of measurement for resistance.
  3. Turn off power to the circuit.
  4. Remove the resistor you want to measure. If you leave the resistor in the circuit you may not get an accurate reading.
  5. Touch the tips of the probes to each side of the resistor.
  6. Read the display, taking care to note the units. A reading of 10 may indicate 10 ohms, 10 kilo-ohms or 10 mega-ohms.

Measuring Voltage

  1. Connect the multimeter to a circuit. Put the black probe in the common terminal and the red probe in the terminal marked for measuring volts and ohms.
  2. Set the multimeter for the voltage you're measuring. You can measure volts DC (direct current), millivolts DC or volts AC (alternating current). If your multimeter has an auto-range function, it's not necessary to select the voltage you're measuring.
  3. Measure AC voltage by placing the probes across the component. It's not necessary to observe polarity.
  4. Observe polarity when measuring DC voltage or millivoltage. Place the black probe on the negative side of the component and the red probe on the positive side.
  5. Read the display, taking care to note the units.
    • If you prefer, you can use the touch-hold feature to keep the reading on the display after you remove the probes. The multimeter will beep each time a new voltage is detected.

Measuring Current

  1. Choose either the terminal marked for measuring 10 amps or the one marked for measuring 300 milliamps (mA). If you're unsure of the current, start in the 10 amp terminal until you're sure the current is less than 300 milliamps.
  2. Set the multimeter to measure current. This may be represented by the letter A.
  3. Turn off power to the circuit.
  4. Break the circuit. To measure current, you must place the multimeter in series with the circuit. Place the probes on either side of the break, observing polarity (black probe on negative side, red probe on positive side.)
  5. Turn the power on. The current will run through the circuit, up the red probe and through the multimeter, then out the black probe and into the circuit.
  6. Read the display, remembering whether you are measuring amps or milliamps. You can use the touch-hold feature if desired.

Testing Diodes

  1. Insert the black probe into the common terminal and the red probe into the terminal marked for measuring ohms, volts or diode test.
  2. Use the selector knob to choose the test diode function. This may be represented by a symbol representing a diode, an arrow pointing at a vertical line.
  3. Turn the power off to the circuit.
  4. Test forward bias. Place the red probe on the positive side of the diode and the black probe on the negative side. If you get a reading of less than 1 but more than 0, the forward bias is good.
  5. Reverse the probes to test reverse bias. If the display reads "OL (overload)," this indicates that reverse bias is good.
  6. A reading of "OL" or 0 while testing forward bias, and 0 while testing reverse bias indicates that the diode is bad.
    • Some multimeters will beep when the reading is less than 1. This doesn't necessarily indicate a good diode, because a shorted diode will also cause a beep.

Measuring Continuity

  1. Put the black probe into the common terminal and the red probe into the terminal used to measure volts and ohms.
  2. Set the multimeter to the same setting you use to test diodes.
  3. Turn the power off to the circuit.
  4. Place the leads on either side of the section of circuit you're testing. It's not necessary to observe polarity. A reading of less than 210 ohms indicates good continuity.



Warnings

  • Never use a multimeter to test whether a high-voltage circuit is dead, on circuits that use more than 4800 watts, on a main power supply to a building, when the multimeter's battery is low or if its probes are damaged. All these conditions represent a significant shock hazard.

Things You'll Need

  • Multimeter
  • Leads
  • Electronic Circuit

Sources and Citations

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